A variety kinds of vehicle power transmission devices for easy driving have been developed in an attempt to facilitate the driving of vehicles and to reduce fatigue to the drivers, and have been employed for the vehicles placed in the market. One of them may be an easy-to-drive device which uses a transmission of the type of a parallel axis gear mechanism similar to that of a so-called manual vehicle combined with an automatic clutch, so that the clutch can be automatically connected or disconnected at the time when the driver attempts to change the gear by shifting the gear by means of a gear-change lever without really using the clutch pedal. There has also been provided a power transmission device which automatically changes the gear depending upon the traveling conditions of the vehicle by using an electronic controller instead of operating the gear-change lever by the driver. As a transmission of a high class, there has further been provided a power transmission device equipped with a manual gear-change mode in which the driver changes the gear and an automatic gear-change mode in which an electronic controller automatically changes the gear, either one of which modes being selected by the driver.
In recent years, there has been developed a power transmission device having a fluid coupling interposed between an engine and an automatic clutch for use in vehicles equipped with a diesel engine. With the fluid coupling being interposed, the vehicle can be started by utilizing slipping between the pump and the turbine in the fluid coupling particularly when the vehicle uses a diesel engine that produces a large torque in a region of low engine rotational speeds. Namely, this easily realizes a smooth start without requiring sophisticated clutch work which is carried out at the time of starting a manual vehicle and, at the same time, absorbs fluctuation in the engine torque during the idling and, further, reduces vibration and noise.
In the power transmission device having the automatic clutch as described above, in general, the engine is equipped with an electronic engine controller. The engine controller stores, for example, a map that determines the amount of fuel supply depending, for example, upon the amount the accelerator pedal is depressed by the driver and the engine rotational speed. When normally traveling, the engine controller controls the amount of fuel supplied to the engine by using the amount the accelerator pedal is depressed as a basic parameter (the engine control for controlling the amount of fuel supply by using the amount the accelerator pedal is depressed as a basic parameter is hereinafter referred to as “accelerator pedal-follow control”). The engine controller is further connected to a clutch controller that controls the clutch; i.e., these two controllers are so linked together as to control the operation condition of the engine depending upon a condition of connection of the clutch at the time of changing the gear.
FIG. 1 schematically illustrates the above vehicle power transmission device. In the power transmission device, a fluid coupling 2 is linked at the back of a diesel engine 1 and a transmission 4 having a parallel axis gear mechanism is further linked thereto via a wet multi-plate clutch 3. An output shaft 41 of the transmission 4 drives the wheels that are not shown of the vehicle. Except when the vehicle is going to start, a pump 21 (integral with an output shaft of the diesel engine 1) and a turbine 22 (integral with an input shaft 31 of the wet multi-plate clutch 3) of the fluid coupling 2 are coupled together by a lockup clutch 23 and, thus, the output shaft of the diesel engine 1 is directly coupled to the input shaft 31 of the wet multi-plate clutch 3. The transmission 4 is an ordinary transmission of the type of a parallel axis gear mechanism in which a gear-change sleeve is brought into mesh with a gear spline that is formed in a gear integrally therewith, and is equipped with a known synchronizing mechanism which may be a synchronizer ring.
The diesel engine is equipped with an engine controller 11 and the wet multi-plate clutch 3 is equipped with a clutch controller 31, and these controllers are so linked together as to control the diesel engine 1 and the wet multi-plate clutch 3. These controllers receive rotational speed signals from a rotational speed sensor 51 that detects the rotational speed of the input shaft 32 of the wet multi-plate clutch 3 (rotational speed of the turbine 22 of the fluid coupling 2), from a rotational speed sensor 52 that detects the rotational speed of the output shaft 33 of the wet multi-plate clutch 3 (rotational speed of the input shaft of the transmission 4) and from a rotational speed sensor 53 that detects the rotational speed of the output shaft 41 of the transmission 4. The controllers use the rotational speed signals for the control operation at the time of changing the gear.
The wet multi-plate clutch 3 in the power transmission device is a general wet multi-plate clutch in which many friction plates spline-fitted to the input shaft 32 and many friction plates spline-fitted to the output shaft 33 are alternately arranged. The amount of connecting the wet multi-plate clutch 3 is controlled by adjusting the hydraulic pressure acting on a piston that pushes the friction plates depending upon a duty ratio D of pulses produced from the clutch controller 31 as taught in, for example, JP-A-2002-295529. Here, the wet multi-plate clutch 3 is completely connected when the duty ratio is 0%, and is disconnected when the duty ratio is 100% since the amount of connection is zero.
Referring to FIG. 6, described below is the operation of the power transmission device at the time of changing the gear for shifting up (fourth speed→fifth speed). At the time of changing the gear, the clutch controller 31 that had been producing a duty ratio of 0% now produces a duty ratio of 100% in response to a gear-change instruction signal A to disconnect the wet multi-plate clutch 3 and, hence, to separate the transmission 4 away from the diesel engine 1. The gear (fourth speed) of the transmission 4 that had been transmitting the power up to now is disengaged to bring the transmission 4 to the neutral. The transmission 4, thereafter, is brought into mesh with a new gear (fifth speed). During this period, the rotational speed of the output shaft of the transmission 4 is brought into synchronism with the rotational speed of the new gear by the synchronizing mechanism, and the mesh with the new gear is accomplished (gear engaged) at a moment the synchronism is obtained and, thus, change of the gear of the transmission 4 is completed. In the case of the manual operation, the gear-change instruction signal A is produced when the driver's attempt for changing the gear is detected by a detector installed in the knob of a gear-change lever 61 that is operated by the driver. When the transmission 4 is in the state of automatically changing the gear relying on an actuator or the like, the vehicle controller judges whether it is necessary to change the gear depending upon the vehicle speed and the amount the accelerator pedal 62 is depressed, and the gear-change instruction signal A is automatically produced when it is necessary to change the speed.
After the gear has been engaged, the clutch controller 31 connects the wet multi-plate clutch 3 so that the output of the diesel engine 1 is transmitted again to the transmission 4. In this case, however, in order to avoid the shock of gear-change caused by a sudden transmission of torque or to avoid the engine stall, the clutch controller 31 works to gradually decrease the duty ratio and to gradually increase the amount for connecting the wet multi-plate clutch 3 as disclosed in, for example, JP-A-2002-295529. When the wet multi-plate clutch 3 is completely connected, the diesel engine 1 is in a state of being directly coupled to the input shaft of the transmission 4 and, thus, changing the gear is finished by the transmission 4 and the wet multi-plate clutch 3. In practice, the wet multi-plate clutch 3 has an invalid region where the amount of connection does not almost increase despite the hydraulic pressure is elevated from the disconnected state. Therefore, the duty ratio is very decreased for a very short period of time so that the invalid region is quickly passed through. Thereafter, the clutch controller 31 produces such a duty ratio that enables the wet multi-plate clutch 3 to be connected in a proper amount.
As shown in the lower portion of FIG. 6, when the wet multi-plate clutch 3 is disconnected in response to the gear-change instruction signal A and the transmission 4 is separated away from the diesel engine 1, the rotational speed of the input shaft of the transmission 4 (output shaft 33 of the wet multi-plate clutch 3) quickly drops down to a rotational speed that corresponds to the new gear (fifth speed) accompanying the operation of the synchronizing mechanism. Concerning the output shaft of the diesel engine 1 (input shaft 32 of the wet multi-plate clutch 3), on the other hand, there takes place a phenomenon of a so-called flare (rev-up of the engine) in which the rotational speed suddenly increases since the diesel engine 1 is suddenly liberated from the load for traveling the vehicle. When this phenomenon takes place, the engine noise increases, a difference in the rotational speed increases between the input shaft and the output shaft of the clutch, and an increased period of time is required for completely connecting the clutch.
To prevent a sudden increase in the rotational speed and to decrease a difference between the rotational speed of the input shaft 32 of the wet multi-plate clutch 3 and the rotational speed of the output shaft 33 thereof, the control mode of the engine controller 11 at the time of changing the gear is changed over to a gear-change engine control that is not dependent upon the amount the accelerator pedal 62 is depressed, and the amount of fuel supplied to the diesel engine 1 is greatly limited, e.g., decreased to an amount of fuel during the idling of the engine. That is, in response to the gear change instruction signal A, the engine controller 11 discontinues the accelerator pedal-follow control and performs a mode of gear-change engine control to control the diesel engine 1 independently of the amount the accelerator pedal 62 is depressed. This greatly decreases the amount of fuel but is still often accompanied, however, by the occurrence of flare due to time delay in the control. The gear-change engine control continues from when the gear-change instruction signal A is input until the connection of the wet multi-plate clutch 3 is completed. Thereafter, the engine control returns back to the normal accelerator pedal-follow control. At the start and at the end of the gear-change engine control, a so-called damping is executed for increasing or decreasing the amount of fuel stepwise for short periods of time to avoid adverse effect caused by a sudden change in the amount of fuel.
At the time of changing the gear for shifting up, the clutch is disconnected, the output of the engine is temporarily shut off, and the vehicle decelerates. Before shifting up, the driver, in many cases, is attempting to accelerate the vehicle by depressing the accelerator pedal. Therefore, a deceleration at the time of changing the gear gives the driver a so-called sluggish feeling and a shock of gear-change, and impairs the drive feeling. With the low gears such as the first speed and the second speed, in particular, the transmission produces a large output torque and the vehicle usually accelerates sharply. Therefore, changing the gear for shifting up with low gears gives the driver a more sluggish feeling. The above deterioration in the drive feeling is called “acceleration spoiling”.
In order to prevent the drive feeling from impaired by the “acceleration spoiling” at the time of changing the gear, the present applicant has developed a gear-change controller as disclosed in JP-A-60-11757. This gear-change controller is “provided with means for judging whether the engine can produce a surplus of acceleration for the horsepower required for traveling the vehicle, and when the change of gear is the shifting up, turns the engine throttle toward the closing direction prior to disconnecting the clutch to decrease the surplus of torque”. That is, when the engine output is considerably greater than the horsepower required for the traveling and the acceleration of the vehicle is exceeding a predetermined amount, the engine output is squeezed for a short period of time prior to disconnecting the clutch for changing the gear. This lowers the acceleration of the vehicle prior to changing the gear and relaxes a sudden “acceleration spoiling”.
The above controller aims at relaxing the “acceleration spoiling” and decreases the engine output prior to disconnecting the clutch for changing the gear. At a moment when the clutch is disconnected, therefore, the engine output has been decreased to a considerable degree, and the engine rotational speed is suppressed from increasing even when the load for traveling the vehicle is removed. Therefore, a control for decreasing the engine output prior to disconnecting the clutch for changing the gear (hereinafter called “pre-reduction control”) is effective in preventing the occurrence of flare at the time of changing the gear.
According to the art disclosed in the above JP-A-60-11757, however, the pre-reduction control that precedes the operation for disconnecting the clutch is executed when the engine output is considerably greater than the horsepower that is required for the traveling and when the acceleration of the vehicle is exceeding a predetermined amount. In other words, the pre-reduction control is not executed when the above conditions are not satisfied, and the flare may occur at the time of shifting up. Further, the amount of returning back the throttle for lowering the engine output is set to be constant. Depending upon the operating conditions, therefore, the effect of the pre-reduction control becomes insufficient, and the rotational speed is not decreased by a proper amount. Or, the pre-reduction control works so effectively that the rotational speed often decreases excessively.